DNA dependent RNA polymerases play a central role in the transcription process. These studies will concentrate on RNA polymerase II, the enzyme responsible for mRNA synthesis in eukaryotic cells. It is specifically concerned with an analysis of mammalian RNA polymerase II structure and a definition of functional domains within the enzyme. Mammalian cells contain two forms of RNA polymerase II, designated IIO and IIA, that differ in the apparent Mr of their largest subunit, IIo and IIa respectively. The C-terminal domain of subunits IIo and IIa consists of 52 repeats of the heptapeptide consensus sequence tyr-ser-pro-thr-ser-pro-ser. This proposal is designed to test the hypothesis that the C-terminal domain of the largest polymerase II subunit is a major factor in determining the selectivity of transcription and that modifications in this region distinguish RNA polymerases IIO and IIA. The specific objectives are as follows: 1) define the role of the largest subunit C-terminal domain in the transcription process, 2) define the structural relationship between RNA polymerases IIO and IIA, with a special emphasis on modifications within the C-terminal domain, and 3) define the transcriptional activity of RNA polymerases IIO and IIA. We hope that these studies will lead to a better understanding of the comple molecular structure of eukaryotic RNA polymerases and of how this complexity relates to polymerase function.